In recent years, an individual identification technology using wireless communication (hereinafter referred to as a wireless communication system) has attracted attention. In particular, as a data carrier which transmits and receives data by wireless communication, an individual identification technology with a wireless tag (hereinafter collectively referred to as a “wireless tag” regardless of the shape such as a card shape or a chip shape) utilizing an RFID (radio frequency identification) technology has attracted attention. The wireless tag is also referred to as an IC tag, an RFID tag, or an electronic tag.
The individual identification technology using the wireless tag has started to be used for management of a large number or a large amount of goods in the field of production and distribution as an alternative to conventional barcode management, and has been developed to be applied to personal authentication.
A wireless communication system herein is a communication system in which data is wirelessly transmitted and received between a transceiver (also referred to as an interrogator) such as a reader/writer (hereinafter referred to as a R/W) and a wireless tag. In such wireless communication, data to be transmitted and received is superimposed on a carrier wave generated from the R/W, whereby data is exchanged.
In some of wireless tags, a DC voltage for driving is generated in the wireless tag by reception of a carrier wave or an amplitude modulation wave generated by superimposing a modulation wave on a carrier wave, from a R/W, and thus, driving electric power of a circuit in the wireless tag is covered. In such a wireless tag, a battery is not provided. Such a wireless tag is generally called a passive type wireless tag. On the other hand, the one covering electric power required for operation as a wireless tag by using a battery incorporated in the wireless tag itself is called an active type wireless tag.
A passive type wireless tag has an advantage in that the size and the weight can be reduced because a battery is not provided; however, by receiving a carrier wave from a R/W, driving electric power is generated, and thus, if the carrier wave is not sufficiently received, electric power required for circuit operation can not be generated.
As factors required in the case where an application is built using such a wireless communication system, a long maximum communication distance, recognition of all the wireless tags in the case where a large number of wireless tags are provided in a specific region, and the like can be given.
Electric power of a carrier wave from a R/W is often increased to achieve the above two factors. Therefore, in the case where a wireless tag is placed very close to the R/W, or in the case where the number of wireless tags which are placed close to the R/W is small, very high electric power is received by the wireless tag.
In the case where excessive electric power is supplied to the wireless tag, a DC voltage which is generated internally is also excessively generated; thus, an element in the wireless tag may be broken because a high voltage is applied to an internal circuit of the wireless tag. Therefore, in a wireless tag with which a wireless communication system whose electric power of a carrier wave from a R/W is high may be constructed, in the case where the wireless tag is present in an environment where excessive electric power is received, an overvoltage protection circuit with which excessive electric power is prevented in the wireless tag may be provided. Such a wireless tag is disclosed in the following Patent Document 1 and Patent Document 2.
[Citation List]
[Patent Document]
    [Patent Document 1] Japanese Published Patent Application No. 2006-180073    [Patent Document 2] Japanese Published Patent Application No. 2007-183790